Hemostasis relies on the complex coagulation cascade, wherein a series of events mediated by blood clotting factors lead to conversion of prothrombin to thrombin. Factor X (FX) activation is the central event of both the intrinsic and extrinsic pathways of the coagulation cascade. The extrinsic pathway has been proposed as the primary activator of the coagulation cascade (Mackman et al., Arterioscler. Thromb. Casc. Biol., 27, 1687-1693 (2007)). Circulating Tissue Factor (TF) and activated Factor VII (FVIIa) interact to form the “extrinsic complex,” which mediates activation of FX. The coagulation cascade is amplified by the intrinsic pathway, during which successive activation of factors XII, XI, IX, and VIII results in formation of the “intrinsic” FIXa-FVIIIa complex that also mediates FX activation. Activated FX promotes thrombin formation, which is required for the body to create fibrin and effectively curb bleeding.
Severe bleeding disorders, such as hemophilia, result from disruption of the blood coagulation cascade. Hemophilia A, the most common type of hemophilia, stems from a deficiency in factor VIII, while hemophilia B is associated with deficiencies in Factor IX (FIX). Hemophilia C is caused by a deficiency in Factor XI (FXI) (Cawthern et al., Blood, 91(12), 4581-4592 (1998)). There is currently no cure for hemophilia and other clotting diseases. Factor replacement therapy is the most common treatment for blood coagulation disorders. However, blood clotting factors typically are cleared from the bloodstream shortly after administration. To be effective, a patient must receive frequent intravenous infusions of plasma-derived or recombinant factor concentrates, which is uncomfortable, requires clinical settings, is expensive, and is time consuming. In addition, therapeutic efficacy of factor replacement therapy can diminish drastically upon formation of inhibitory antibodies. Approximately 30% of patients with severe hemophilia A develop inhibitory antibodies that neutralize Factor VIII (FVIII) (Peerlinck and Hermans, Haemophilia, 12, 579-590 (2006)). Few therapeutic options exist for patients with anti-Factor antibodies.
Thus, there exists a need in the art for compositions and methods for treating blood coagulation disorders. The invention provides such compositions and methods.